1. Field of the Invention
The present invention relates to a method of differentiating a type of heat sensitive paper. More particularly, the present invention relates to a method of differentiating a type of heat sensitive paper stacked in a thermal printer.
2. Description of the Related Art
There are two types of thermal printers, direct thermal printers and thermal transfer printers. Direct thermal printer uses paper which is color-developed based on the heat of a thermal head (referred to as “heat sensitive paper”). The thermal transfer printer uses an ink ribbon that responds to heat and transfers ink from the ink ribbon to conventional paper. Since the thermal transfer printer has to employ a driving unit for driving the ink ribbon, its construction is complicated and expensive. Also, since the ink ribbon is consumable, it must be continuously replaced. It increases the per sheet printing costs.
Referring to FIG. 1, heat sensitive paper 10 comprises a base sheet 11 and first and second ink layers 12 and 13 of a desired color formed on both surfaces of the base sheet 11. The ink layers 12 and 13 may have a single-layer structure of monochromatic ink or a multilayer structure capable of developing at least two colors. For example, the first ink layer 12 is layered with two layers of magenta and yellow, while the second ink layer 13 has a single-layered structure for developing a cyan color. Preferably, the base sheet 11 comprises a transparent material. One example of the heat sensitive paper 10 is disclosed in U.S. Patent Application Publication No. 2003/0125206, which is incorporated herein by reference.
The thermal printer using the heat sensitive paper 10 comprises a thermal printhead (TPH) with heating elements arranged in a direction perpendicular to a feeding direction of paper. In order to print double sides using the single thermal printhead TPH, a first surface of the paper is printed, and then a second surface of the paper is printed by the thermal printhead. If both surfaces are printed, a color image can be seen on the paper, when viewed from one surface of the heat sensitive paper.
FIG. 2 is a schematic view depicting a construction of a conventional thermal printer.
Referring to FIG. 2, the thermal printer comprises a feeding roller 2 for transferring the heat sensitive paper 10, a platen roller 3 for supporting one surface of the paper 10, and a thermal printhead 4 for forming an image on the paper 10 supported by the platen roller 3. The paper 10 passing through the feeding roller 2 and an idle roller 5 is pressed towards the feeding roller 2 by the idle roller 5.
Meanwhile, the heat sensitive paper 10 can have different characteristics depending upon the manufacturer or the date of manufacture. The same manufacturer may manufacture different types of paper according to a desired printing quality. In particular, a thermal profile transferred to the heat sensitive paper from the thermal printhead may be different according to the type of heat sensitive paper used to obtain the best print quality.
Therefore, there is a need of a method for differentiating the type of heat sensitive paper in a thermal transfer printer.